The present invention relates to catalytic reforming using a catalyst comprising zeolite L. More particularly, the present invention pertains to use of such catalyst in a conventional gas or oil fired furnace.
Reforming embraces several reactions, such as dehydrogenation, isomerization, dehydroisomerization, cyclization and dehydrocyclization. In the process of the present invention, aromatics are formed from the feed hydrocarbons to the reforming reaction zone, and dehydrocyclization is the most important reaction.
U.S. Pat. No. 4,104,320 to Bernard and Nury discloses that it is possible to dehydrocyclize paraffins to produce aromatics with high selectivity using a monofunctional non-acidic type-L zeolite catalyst. The L zeolite based catalyst in '320 has exchangeable cations of which at least 90% are sodium, lithium, potassium, rubidium or cesium, and contains at least one Group VIII noble metal (or tin or germanium). In particular, catalysts having platinum on potassium form L-zeolite exchanged with a rubidium or cesium salt were claimed by Bernard and Nury to achieve exceptionally high selectivity for n-hexane conversion to benzene. As disclosed in the Bernard and Nury patent, the L zeolites are typically synthesized in the potassium form. A portion, usually not more than 80%, of the potassium cations can be exchanged so that other cations replace the exchangeable potassium.
Later, a further important step forward was disclosed in U.S. Pat. Nos. 4,434,311; 4,435,283; 4,447,316; and 4,517,306 to Buss and Hughes. The Buss and Hughes patents describe catalysts comprising a large pore zeolite exchanged with an alkaline earth metal (barium, strontium or calcium, preferably barium) containing one or more Group VIII metals (preferably platinum) and their use in reforming petroleum naphthas. An essential element in the catalyst is the alkaline earth metal. Especially when the alkaline earth metal is barium, and the large-pore zeolite is L-zeolite, the catalysts were found to provide even higher selectivities than the corresponding alkali exchanged L-zeolite catalysts disclosed in U.S. Pat. No. 4,104,320.
These high selectivity catalysts of Bernard and Nury, and of Buss and Hughes, are all "non-acidic" and are referred to as "monofunctional catalysts". These catalysts are highly selective for forming aromatics via dehydrocyclization of paraffins.
Having discovered a highly selective catalyst, commercialization seemed promising. Unfortunately, that was not the case, because the high selectivity, L-zeolite catalysts did not achieve long enough run length to make them feasible for use in catalytic reforming. U.S. Pat. No. 4,456,527 discloses the surprising finding that if the sulfur content of the feed was reduced to ultra low levels, below levels used in the past for catalysts especially sensitive to sulfur, that then long run lengths could be achieved with the L-zeolite non-acidic catalyst. Specifically, it was found that the concentration of sulfur in the hydrocarbon feed to the L-zeolite catalyst should be at ultra low levels, preferably less than 100 parts per billion (ppb), more preferably less than 50 ppb, to achieve improved stability/activity for the catalyst used.
It was also found that L zeolite reforming catalysts are surprisingly sensitive to the presence of water, particularly while under reaction conditions. Water has been found to greatly accelerate the rate of deactivation of these catalysts. U.S. Pat. No. 4,830,732 discloses the surprising sensitivity of L zeolites to water and ways to mitigate the problem.
U.S. Pat. No. 5,382,353 and U.S. Pat. No. 5,620,937 to Mulaskey et al. disclose a zeolite L based reforming catalyst wherein the catalyst is treated at high temperature and low water content to thereby improve the stability of the catalyst, that is, to lower the deactivation rate of the catalyst under reforming conditions.
Also, several patents and patent applications of RAULO (Research Association for Utilization of Light Oil) and Idemitsu Kosan Co. have been published relating to use of halogen in L-zeolite based monofunctional reforming catalysts. Such halogen containing monofunctional catalysts have been reported to have improved stability (catalyst life) when used in catalytic reforming, particularly in reforming feedstocks boiling above C.sub.7 hydrocarbons in addition to C6 and C7 hydrocarbons. In this regard, see EP 201,856A; EP 498,182A; U.S. Pat. No. 4,681,865; and U.S. Pat. No. 5,091,351.
EP 403,976 to Yoneda et al., and assigned to RAULO, discloses the use of fluorine treated zeolite L based catalysts in small diameter tubes of about one-inch inside diameter (22.2 mm to 28 mm in the examples). Heating medium proposed for the small tubes were molten metal or molten salt so as to maintain precise control of the temperature of the tubes. Accordingly, EP 403,976 does not teach the use of a conventional type furnace or conventional type furnace tubes. Conventional furnaces for catalytic reforming have tubes of usually three or more inches in inside diameter (76 mm or more), whereas EP 403,976 teaches that using tubes having an inside diameter greater than 50 mm is undesirable. Also, conventional furnaces are heated using gas or oil fired burners.
Typical catalytic reforming processes employ a series of conventional furnaces to heat the naphtha feedstock before each reforming reactor stage, as the reforming reaction is endothermic. Thus, in a three-stage reforming process, the overall reforming unit would comprise a first furnace followed by a first-stage reactor vessel containing the reforming catalyst (over which catalyst the endothermic reforming reaction occurs); a second furnace followed by a second-stage reactor containing reforming catalyst over which the reforming reaction is further progressed; and a third furnace followed by a third-stage reactor with catalyst to further progress the reforming reaction conversion levels.
For example, U.S. Pat. No. 4,155,835 to Antal illustrates a three-stage reforming process, with three furnaces (30, 44, 52) and three reforming reactors (40, 48, 56) shown in the drawing in Antal. Example reforming reactors used according to the prior art are shown, for instance, in U.S. Pat. No. 5,211,837 to Russ et al., particularly the radial flow reactor shown in FIG. 2 of Russ et al.
In some catalytic reforming units, as many as five or six stages of furnaces followed by reactors are used in series for the catalytic reforming unit.